Orthogonal frequency division multiplexing (OFDM) has been widely employed in RF wireless communication systems such as wireless cellular systems, digital audio and video broadcasting systems due to its desirable spectral efficiency, easy implementation, and robustness to multi-path propagation and phase distortion. Recently OFDM has been proposed for use in optical communication systems for example, to combat modal dispersion in multimode fiber and chromatic dispersion in single mode fiber.
As is known, there are two types of optical OFDM. One is direct-detection optical OFDM, which uses optical intensity modulation and direct-detection, and the other is coherent optical OFDM, which requires optical IQ modulation and optical coherent detection.
As the symbol rate of an OFDM signal is very low, polarization mode dispersion (PMD) does not cause any significant inter-symbol interference (ISI) in an optical OFDM system. Unfortunately however, PMD causes the state of polarization (SOP) to change with frequency. This SOP misalignment between sub-carriers and carrier induces signal fading, resulting in performance penalties for an optical OFDM system.
To compensate this effect, polarization diversity has been proposed for coherent optical OFDM systems (See, e.g., W. Shieh, W. Chen and R. S. Tucker, Electron Lett., vol. 42, no. 17, 2006). Fortunately, with a direct-detection optical OFDM system, the PMD penalties can be reduced by aligning the SOP of the carrier with the sub-carriers—as in a sub-carrier multiplexing system. Unfortunately however, this method requires dynamic polarization control and cannot completely eliminate the PMD impairments as the SOP misalignment among sub-carriers is not corrected.